Thermal Testing Methods for Materials Evaluation: Techniques, Principles, and Applications, Lecture notes of Materials science

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MM 212

Materials Evaluation Techniques

Fall Semester 2018, FMCE, GIKI

Instructor: Muzammil Irshad Research Associate

Lecture 41, 42 : Thermal Testing Methods

Introduction :

 local differences in thermal properties lead to measurable differences in temperature across a surface.

 Thermal testing give rapid results and are often noncontact (contact methods are also used).

 These methods are better at detecting flaws in thin objects rather than thick objects.

 Not so good at detecting deep-lying flaws.

 Useful for composites or other structures made of dissimilar materials (where the variations in thermal properties are easily detectable)

Thermal conduction:

 Heat flows when a material is subjected to a temperature gradient.

 Thermal energy is transferred by three main mechanisms: conduction, convection, and radiation.

 In the case of materials evaluation, the main interest concerns the utilization of variations in thermal conduction.

 Heat flux (thermal energy per unit area per unit time) obeys Fick’s first law,

 Heat flux depends on the temperature gradient and the thermal conductivity.

 When a material subjected to a heat pulse, the transient temperature in the vicinity of flaws will be different from the parent material. where φ is the heat flux, κ is the thermal conductivity, and dT/dx is the thermal gradient.

Thermal radiation

 Convenient ways to heat a test material for NDT evaluation purposes is by exposure to infrared (thermal) radiation.

 This is radiation with wavelengths greater than 750 nm.

 Near-infrared range has wavelengths up to 10 μm.

 The intensity of infrared radiation from a material depends largely on two Factors (temperature of the material and its surface roughness)

 The wavelength of the infrared radiation from a material has a peak intensity that can be related directly to the temperature (Wien’s displacement law)

where λmax is the wavelength of maximum emittance, T is the temperature in degrees Kelvin, and b is the Wien displacement

coefficient, which is 2,897 μm ⋅ K−1.

Heat flow:  Heat flow is a diffusion process, and under steady state conditions it obeys Fick’s first law.

 Under transient conditions, it obeys Fick’s second law.

 If heat flows perpendicularly through a layer of material with thickness L and thermal diffusivity D the transient temperature on the far side of the layer changes according to Fick’s second law is obtained by above equation:

 The time t1/2 needed for the temperature of the far surface to change by half of this difference, ( Tf − Ti)/2, is:

 Typical values of the thermal diffusivity are D = 100. 10−6 m2 ⋅ sec−1 for

metals, and D = 1. 10−6 m2 ⋅ sec−1 for composites.

 where t is time and x is position

Thermal waves:

 Thermal wave interferometers comprise AC techniques which use phase sensitive detection with a lock in amplifier.

 A modulated laser beam is often used to heat the surface of the test material.

 The thermal waves can be described in a lossless situation by the following equation

 However, in a material there will be some decay of the thermal wave with distance, according to the more general equation.

Thermal inspection procedures:

 There are two main classes of thermal inspection techniques:

(1) Thermography, (which maps temperature variations over a surface)

(2) Thermometry (which is aimed at actual measurement of temperature)

 The normal procedure is to heat the test material, but in some cases cooling is used.

 These normally involve:

(1) Thermal pulses (Laser pulse heating )

(2) Continuous thermal waves, respectively.

Contact detection methods:

 These include measurements using thermocouples, thermistors.

 Thermographic methods which can include the use of coatings, paints, phosphors, and liquid crystals.

 Surface coating methods include color change coatings, phosphor coatings, melting point coatings, and thermochromic liquid crystals (which give different colors across the whole visible spectrum at different temperatures).

Noncontact detection methods:

 These include thermometric techniques such as radiometers and pyrometers, and thermographic methods such as thermal cameras and infrared sensitive films.

Noncontact Thermometry:

 These methods rely on the response of a sensor to infrared radiation.

 They are useful for remote sensing of surface temperatures and particularly for imaging.

(1) Radiometers are thermal sensors: (Mounted in a cavity such that the cavity focuses the heat radiation on the sensor).

(2) Pyrometers, also known as infrared thermometers used for higher temperatures, typically above 700°C. (Radiation is focused on to a sensor such as a thermocouple or thermopile, and the resulting voltage is measured).

Thermography:

A thermograph is a map of temperature across a surface

 Thermography can be used to detect variations in temperature across hot spots or just to locate them (used even for stress Detection)

 The image is often produced using an infrared Camera

 The mechanical inhomogeneities lead to contrast in the resulting image.

 The instrumentation requirements include a radiative heating device under computer control, which gives a transient heat pulse.

 Pulse thermography is the best known method.

Heat Sensitive Paints

Heat sensitive paints can be used over the temperature range 40 to 1600 °C. They can give temperature determination with an accuracy of ± 5 °C.

Heat Sensitive Papers

Heat sensitive papers are also used for temperature detection.

These papers are bonded onto the surface of the test material.

They consist of a wax embedded in a paper substrate.

The wax melts when a particular temperature has been reached.

Noncontact thermography:

In noncontact thermography, sensors are used to detect infrared radiation, which is then converted to a voltage signal and plotted as a function of position over a surface.

Applications of thermography:  Mostly thermography is used for nonmetallic or poorly conducting materials, particularly composites and laminates

 The technique also finds widespread applications:  Plate glass, automobile windshields,  Nonmetallic coatings (uniformity tests), plastics and polymers  Paper rolling and drying.

Sonic infrared inspection ( vibro-thermography) :

 Uses high power ultrasonic excitation and infrared detection to locate defects

 The basic idea is that when a material is subjected to a short (∼1 sec) intense burst of ultrasound (at 20 to 40 kHz), any cracks in the material will move, inducing frictional heating.

 Ultrasonic frequencies excites a larger fraction of the vibrational modes of the specimen  Alternative to both fluorescent particle inspection and magnetic particle inspection.